446 



NATURE 



[SlirTEMBEK 2 2, 1 923 



the common rat-flea was Pvlex (XettopsyUa) cheopis, 

 that it was present in houses in which rats had died 

 of plague and in which some of the residents had 

 become infected, that the pla^^ue-bacillus could multiply 

 in the stomach of this flea, and that the flea would — 

 in the absence of its usual host— attack man. These 

 observations pointed to the importance of this flea in 

 the dissemination of i>lague, and the Second Plague 

 Commission, which was appointed and began work 

 in 1905, definitely proved that Xenopsylla cheopis is 

 the transmitter of the plague-organism from rat to rat 

 and from rat to num. 



The mechanism of transmission of the plague- 

 bacillus was worked out by Bacot and Martin in 1913. 

 They showed that in a proportion of the fleas fed on the 

 blood of septicemic mice the plague-bacilli multiply 

 in the provcntriculus — which is provided with chitinous 

 processes that act as a valve to prevent regurgitation 

 of the blood from the stomach — and a mass of bacilli 

 is formed which blocks the provcntriculus and may 

 extend forward into the oesophagus. Fleas in this 

 condition are not prevented from sucking blood, 

 because the pharynx is the suctorial organ, but their 

 attempts to obtain blood result only in distending the 

 oesophagus. The blood drawn into the oesophagus is 

 repeatedly forced backwards into contact with the 

 mass of plague-bacilli, and on the sucking action 

 ceasing some of this infected blood is expelled into the 

 wound. The transmission of plague depends on the 

 peculiar structure of the provcntriculus of the flea and 

 on the extent to which, in certain examples, the plague- 

 bacilli multiply in the provcntriculus. Such " blocked " 

 fleas being unable to take blood into the stomach are 

 in a starved condition, and make repeated attempts to 

 feed, and hence are particularly dangerous. 



Until 1 9 13 it was believed that all the fleas of the 

 genus Xenopsylla found on rats in India belonged to 

 one species {cheopis), but in that year L. F. Hirst 

 reported that the rat-flea of Colombo was X. astia, 

 which had been taken off rats in Rangoon, and described 

 by N. C. Rothschild in 191 1. Hirst ascertained that 

 this flea did not readily bite man if the temperature 

 were above 80° F. A collection of 788 fleas from 

 Madras City proved to consist entirely of X. astia, and 

 Hirst suggested that the explanation of the immunity 

 of Madras and Colombo from plague was the relative 

 inefficiency of X. astia as a transmitter. Cragg's 

 examination (1921, 1923) of 23,657 fleas obtained from 

 rats in all parts of India shows that they include three 

 species : Xenopsylla cheopis, X. astia, and X. brasili- 

 ensis. This last species is common in the central and 

 northern uplands of peninsular India, but its bionomics 

 have not yet been investigated. X. cheopis is the 

 predominant species in the plague areas, while X. astia 

 is the common flea in those areas which have remained 

 free from plague or have suffered only lightly. In 

 Madras City, for example, during the twenty-one years, 

 1897-1917, plague has occurred in twenty of these 

 years, but the average mortality was only 0-013 P^"" 

 thousand — that is, though the infection has been 

 repeatedly introduced there, it failed each time to 

 set up an epidemic. The significance of an imported 

 case of plague depends in large measure on the local 

 species of Xenopsylla. Hirst has made numerous 

 attempts during the plague season in Colombo to 



NO. 2812, VOL. I 12] 



transmit plague by means of X. astia from rat to rat, 

 but with negative results, and X. astia was never found 

 to l)ehave like a *' blocked " A', cheopis. 



The distinction of X. cheopis from A', astia is init 

 an entomological refinement with purely systematic 

 significance, but corresponds with a different relation 

 of the species to the epulemiology of plague, and hence 

 becomes a factor of great practical importri'^- • " 

 through these researches it has l)ecome po 

 examination of the rat-fleas of a locality to < ;i: ■ 

 accurately its liability to plague, anti-plague imi- iri- 

 may henceforward be restricted to those areas in which 

 plague is likely to occur, i.e. where X. cheopis is the 

 predominant flea. Thus a great economy 

 and of expenditure and a higher degree of < 

 may be achieved ; in fact, the problem of the preven- 

 tion or reduction of plague may be brought from un- 

 wieldy to practicable proportions. When it is remem- 

 bered that since 1896 some ten and a quarter millions 

 of people have died in India from plague, we have a 

 more than sufficient index of the importance of a precise 

 knowledge of the systematics, structure, and bionomi' ^ 

 of the insect-carrier of Bacillus pestis. 



Another of the outstanding features ■ riud 



under review has been the extensive and uiicnsive 

 study of the Protozoa. The structure and the bio- 

 nomics and life-history of these organisms have iRcn 

 investigated with the help of the finest developments 

 of modern technique. It is fitting here to record our 

 acknowledgment to two staining methods — Heiden- 

 hain's iron-haematoxylin and the Romanowsky stain 

 (including Giemsa's and Leishman's modifications), 

 which have added greatly to our technical resources. 



There is time to refer only to certain of the Protozoa 

 which directly affect man. Twenty years ago our 

 knowledge of the few species of Protozoa recorded 

 from the human alimentary canal was defective in 

 two important respects — the systematic characters 

 and the biology of the species — so there was much 

 confusion. Subsequent investigations, and esjjecially 

 those of the last ten years (by Wenyon, Dobell, and 

 others), have cleared up most of the doubtful points, 

 but owing to the difficulties of size and the paucity of 

 characters available, it is by no means easy in practice 

 to distinguish certain of the species. Of the seventeen 

 species now known to occur in the intestine of man, 

 Entamoeba histolytica has received particular attention. 

 This organism lives as a tissue parasite in the wall of 

 the large intestine, where, as a rule, the damage caused 

 is counterbalanced by the host's regenerative processes. 

 But when the destruction outstrips the regeneration 

 intestinal disturbance results, leading to the condition 

 known as amoebic dysentery. The specific characters 

 and the processes of reproduction and encystment of 

 E. histolytica are now well ascertained, and it is realised 

 that in the majority of cases the host is healthy, acting 

 as a " carrier " dangerous to himself, for he may develop 

 into a case of acute dysenter}-, and to the community — 

 for he is passing in his faeces the encysted stage which 

 is capable of infecting other persons. \\Tiether an 

 infected person will sufl[er from dysenter\- or act as a 

 healthy " carrier " apparently depends upon his own 

 susceptibility rather than on any difference in the 

 virulence of different strains of the Entamoeba. 



In all work with Entamoebae infecting human beings 



